In its simplest form, concrete is a mixture of paste and aggregates, or rocks. The paste, composed of portland cement and water, coats the surface of the fine (small) and coarse (larger) aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to form the rock-like mass known as concrete. Within this process lies the key to a remarkable trait of concrete: it is malleable when newly mixed, strong and durable when hardened. These qualities explain why concrete can be suitable for building skyscrapers, bridges, sidewalks and superhighways, houses and dams.
A concrete block is often used as a building material in the construction of walls. It is sometimes called a concrete masonry unit (CMU). A concrete block is one of several precast concrete products used in construction. Others include segmental retaining walls (SRW), interlocking concrete pavers (ICP), veneer, and a host of other concrete products. The term “precast” refers to concrete products that are formed and hardened before they are brought to the job site. Most concrete blocks have one or more hollow cavities, and their sides may be cast smooth or with a design. In use, concrete blocks are stacked one at a time and may be held together at joints between the units with fresh concrete mortar to form the desired length and height of the wall. Additional load resistance is supplied by placing reinforcing steel (rebar, anchorage, etc.) and grout within the grout spaces of wall assemblies. Some applications may not use mortar and/or rebar but other means to reinforce and stabilize including friction, pins, geogrid, special sands, etc. After the concrete is thoroughly mixed (by mixing portland cement, aggregates, and water) and workable, it can be placed in forms before the mixture becomes too stiff. Through a series of chemical reactions including hydration, a cement paste develops then hardens and gains strength to form a matrix around the aggregates. During these chemical reactions, crystals, nodes, and other forms additionally develop on and within the paste as well as on the surfaces of aggregates or other portions of the concrete particles. In other words, the initial dry cement powder which comprises finely ground cement clinker, after exposure to sufficient water and mixing, changes into different chemicals and also while doing so the physical nodes in the paste grow and expand. Often they interconnect within the complex paste matrix. These reactions may also include aggregates such as mechanically interlocking as well as chemically interacting with the surfaces of aggregate particles.
In the production of precast concrete products (e.g., such as precast concrete blocks), the concrete is discharged into molds (for shaping the blocks) while the concrete mixture is still loose and “fresh.” In many circumstances, it is desirable to minimize time that the concrete must remain in the mold before it is sufficiently hardened such that it will not “slump” when removed from the mold. Thus, the concrete mixture can sometimes be provided into the mold as a “zero-slump concrete” or “near-zero slump concrete,” which has a sufficiently high consistency and/or set time to allow forming into unit configurations such as by a block-making machine. Once formed the then unit-configured concrete is ejected from the mold for subsequent curing.
When the concrete blocks are initially removed from the mold (referred to as “green” units or “wet” units), they are in the form of precast wet concrete products that are then ready to be placed in a curing rack, which may hold several hundred wet units; the racks holding the pallets in turn are moved into a curing chamber. Normally, an individual curing chamber (frequently called a “kiln”, though not operating at high temperatures) takes about two hours if not more time to “charge” or load. Some curing chambers can be built to a much larger capacity and continually accept new, “green” or “wet” product until the end of a production shift. Additionally it is traditionally assumed to incorporate a long “preset” time—also measured in hours such as 4-5 hours—once the curing chamber is charged (filled), for the just-formed wet units to set and then begin hardening, Curing can ensure the continued hydration of the accessible portions of cement within the paste so that the concrete continues to gain strength. There are several basic types of curing chambers or kilns. A common type is a “low-pressure steam” curing chamber. In this type, the blocks are held inside without adding moisture during preset at ambient temperature to allow them to set and then harden slightly. A heavy steam is then gradually introduced to raise the temperature at a controlled rate of not more than approximately 60° F. per hour (33° C. per hour). When the peak curing temperature has been reached and sufficient “soak” time has been attained, the steam is shut off. The blocks are then partially dried either by exhausting the moist air or by simply extending the curing time until removal. The whole curing cycle takes about 24 hours to several days. Alternately, certain climates allow for no additional use of steam as a heat and moisture source within the curing chamber and rather take advantage of one of the properties of cement hydration, that being the “heat of hydration.” The exothermic reaction during cement hydration contributes to certain economies of curing that are usually not available in colder climates. However there may still be unresolved color, efflorescence, and other aesthetic and environmental issues.
Another type of kiln is the “high-pressure steam” kiln, sometimes called an autoclave. In this type, the temperature is raised to approximately 375° F. (191° C.), while the pressure is also raised to approximately 116 to 174 psi (8 to 12 bar). The blocks are allowed to heat soak for about 5-12 hours. The pressure is then rapidly vented, which causes the blocks to quickly release their trapped moisture. The autoclave curing process requires more energy and a more expensive kiln, but it can produce blocks lighter in weight though with less compressive strength.
The quality of the cement and the amount and type of aggregate, which are implemented during the mixing process before discharging the mixture into the mold, are some of the controlling features affiliated with raw materials that can dictate the character of the concrete block. Conventionally, in a traditional zero-slump to near-zero-slump concrete mixing method for a normal weight density mix, the water-cement ratio may be traditionally minimized to an approximately range between 0.35 to 0.50 by weight so as to ensure desired formability, strength, and aesthetic properties. (This traditional amount of water, based upon traditional batch methods, is referred to as “100% of the total assumed batch water.”) Yet due to unprotected aggregates received or becoming wet, a 6,500 pound normal-weight batch may only take 40 pounds of added water or less, added often primarily toward the end of the batch cycle rather than the beginning of the batch cycle. There are usually narrow limits to how much water a certain mix can accept in a traditional mixing process before undesirable properties appear. Water may be further limited by modern water reducing admixtures, which when used do tend to assist in the speed of strength development but at a cost of increasing the amount of cement per batch and/or other compromises. The aesthetic surface feature of “swipe,” which gives a glossier and pasty smear look to the finish, is traditionally accomplished by including synthetic plasticizing admixtures and/or additional water. However, adding excess water in a traditional mixing process can reduce the consistency of the zero-slump to near-zero-slump concrete mixture prior to discharging the mixture into the mold. As such, the unit shape stability, setting, and hardening characteristics may be affected. Excess moisture in a traditional mixing process may also result in a need for longer residence time in the mold and/or undesired slumping or deforming of the concrete products after the mold, longer curing time, increased curing energy burdens, strength reduction, unacceptable aesthetic texture, color, and/or uneven compression banding.
As to the amount of cement used within a batch during the mixing process (before discharging the mixture into the mold), one traditional long-term average for zero-slump to near-zero-slump concrete mix is about 12.4% cement of the aggregate by weight using traditional methods (excluding hardscape products).